The objectives of this research proposal are to identify the details of the involvement of the vitamine D endocrine system, particularly 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] in the secretion of insulin by the pancreatic B-cell. The program will be carried out at three levels: (A) Studies in intact vitamin D-deficient and replete rats and rabbits and to determine the dose response and time course requirement for in vivo 1,25(OH)2D3 administration in the secretion of insulin by (i) subsequently isolated and perfused pancreases and (ii) their response to a standardized glucose tolerance test. (B) Studies with isolated insulin secreting cells (obtained with a flow cyto-meter) from D-replete and D-deficient animals which will (i) quantitate the 1,25(OH)2D3 receptor and vitamin D-dependent calcium binding protein (CaBP) and (ii) determine the response of these cells to in vivo and in vitro 1,25(OH)2D3 addition on 45Ca2+ uptake and release and insulin release. (C) Biochemical studies on pancreatic CaBP to (i) learn its molecular weight and Ca2+ binding properties (ii) determine using cDNA probes whether it is synthesized de novo in the B-cell and (iii) to study via a photoaffinity probe as to whether the CaBP has a preferential interaction with any cellular constituents of the insulin secreting cells. Collectively these studies will allow formulation of both a physiological as well as molecular model to describe the involvement of 1,25(OH)2D3 in the secretion of insulin.